The present invention relates to apparatus for monitoring shaft voltage and current in large dynamoelectric machines and, more particularly, to quick-release and quick-clamping apparatus for affixing pickup braid of a shaft voltage and current monitoring system in position contacting the shaft of a large dynamoelectric machine.
The shaft of a large dynamoelectric machine such as, for example, a large generator of a steam turbine-generator system is electrically insulated from the generator frame by, among other things, a lubricant film in conventional journal bearings. Several sources may produce a voltage on the shaft which, if allowed to rise to a sufficiently high value, may produce arcing through the lubricant film resulting in pitting of the bearing surfaces and a resultant acceleration in bearing wear.
A static voltage may be generated in the shaft by condensed water droplets being thrown off the buckets of the last stage in the steam turbine and carrying electric charges with them. Although this source does not provide for a large current, if satisfactory means is not provided for bleeding off the static voltage it may rise to a high enough value to arc through the bearing lubricant film.
An AC ripple on the DC field winding of the generator rotor may be capacitively coupled to the rotor forging and thence to the shaft to produce an AC voltage thereon. This is also a weak source of voltage which will decay rapidly and/or remain at a low level if current is allowed to flow through an appropriate grounding apparatus.
It is conventional to provide a grounding apparatus on the turbine end of the generator shaft to dissipate voltage from the above two voltage sources. In order to verify the operation of the grounding equipment, it is useful to monitor the voltage on the generator shaft. Since the voltage from the above two sources is easily discharged, monitoring the shaft voltage requires high-impedance voltage monitoring equipment to prevent the voltage from decaying through the input impedance of the voltage monitoring equipment.
A substantial voltage may be generated between the ends of the generator shaft due to dissymmetries in the generator magnetic circuits. In contrast to the above-described sources, the voltage difference between the ends of the generator shaft, if it finds a suitable path, is capable of supplying a very substantial current. The collector end of a generator shaft is usually well insulated from the generator frame and thus limits the current normally expected to flow through the bearings and generator frame to a very low value. If the insulation at the collector end of the generator shaft breaks down, or becomes ineffective, relatively large currents, in the range of several amperes, can flow through the grounding apparatus at the turbine end of the generator shaft with very rapid deterioration of the affected bearings. It is therefore desirable to monitor the current flowing in the grounding apparatus to detect the large currents which may occur with this type of fault.
Conventional grounding apparatus employs a plurality of sliding contact devices contacting the turbine end of the shaft both for grounding and for monitoring of voltage and current. Such sliding contact devices include, for example, carbon brushes, copper braid and silver-plated copper braid. Due to its low cost and the low-resistance contact that copper braid makes with a steel shaft, such copper braid is favored as sliding contact devices in a number of applications. Typically, such copper braid is clamped in bolted clamping devices with a substantial length of the copper braid in trailing contact with the surface of the shaft.
After a period of use, the contact resistance of such copper braid increases due to pickup of dirt, oil and grease and the oxidation of the surface of the copper. The desired low contact resistance may be restored by removing and cleaning or replacing the copper braid.
A large steam turbine-generator system remains on line at full operating speed for long periods of time and the process and risk of stopping one for any reason are far from trivial. The copper braid and the clamping device holding it in position are necessarily in close proximity to the rapidly rotating generator shaft and must usually be removed and replaced while the generator continues in operation. The requirement for removing a plurality of bolts to remove a braid and for installing and tightening such bolts for replacing the braid, all performed in close proximity to a rapidly rotating (e.g. 3600 RPM) shaft and a bolted coupling is, at the least, a cause for extra care and possible concern on the part of a worker having the responsibility to do this job.